Currently, wavelength division multiplexing (WDM) has been widely used, which multiplexes and sends signal light having different wavelengths to increase the transmission capacity. In optical ring networks, optical transmission apparatuses having an optical add-drop multiplex (OADM) function for adding or dropping a signal with a wavelength.
In such optical transmission apparatuses, even when the level of light per wavelength is as low as several dBm, if signal light is multiplexed, a very high output power of 20 dBm, for example, is obtained, which needs safety precautions. For example, some optical transmission apparatuses fall in CDRH (Center for Devices and Radiological Health) Class IIIb, a safety precaution standard. Therefore, optical transmission apparatuses are generally provided with an automatic power shut down (APSD) function as a safety precaution. When an unusual connection of optical fiber is detected, the laser diode (LD) of the optical amplifier is shut down to forcibly stop outputting the signal light.
FIG. 9 illustrates an example optical ring network. As illustrated in FIG. 9, optical transmission apparatuses 101 to 106 form a ring network with an upstream link 111 and a downstream link 112. In the ring network, the optical transmission apparatus 101 adds signal light, and the optical transmission apparatus 104 drops the signal light. The optical transmission apparatus 101 outputs the identical signal light to the upstream link 111 and the downstream link 112, and the optical transmission apparatus 104 drops the signal light from the upstream link 111.
It is assumed here, for example, that the optical fiber was disconnected in the upstream link 111 between the optical transmission apparatus 102 and the optical transmission apparatus 103. The optical transmission apparatus 103 detects the disconnection of signal light and forcibly shuts down the output of the signal light from the optical transmission apparatus 102 (APSD function). The optical transmission apparatus 104 switches a port for dropping signal light from the upstream link 111 to the downstream link 112. With this switching, even though the optical fiber was disconnected in the upstream link 111 between the optical transmission apparatus 102 and the optical transmission apparatus 103, the transmission of the signal light is maintained.
In such an optical communication network, a state where no operating wavelength light (main signal) is used (only optical supervisory channel (OSC) light is used, hereinafter called a zero-wave operation state) occurs when a protection function (such as port switching described above) is executed, when the number of wavelengths is specified in response to a request in an optical transmission apparatus having the OADM function, or when an optical network is started up. Even in such a zero-wave operation state, the connection of optical fiber, including a dispersion compensation fiber (DCF), needs to be monitored.
The detection of the state of an optical transmission apparatus will be described below at a usual operation state (where operating wavelength light and OSC light are used) and a zero-wave operation state. First, the structure of the optical transmission apparatus will be described.
FIG. 10 is a block diagram of an optical transmission apparatus. FIG. 10 illustrates the optical transmission apparatus 102 illustrated in FIG. 9. The other optical transmission apparatuses have the same structure.
The optical transmission apparatus 102 includes optical amplifier units 120 and 140, OSC units 151 and 152, and core (switch) units 153 and 154. For example, each unit consists of one printed circuit board and is connected to other units through connectors.
The optical amplifier unit 120 amplifies signal light received from the optical transmission apparatus 101 and outputs it to the core unit 153. The core unit 153 drops the signal light or adds signal light, and outputs the signal light to the optical amplifier unit 140 (although the optical transmission apparatus 102 does not drop the signal light or add signal light in the case of FIG. 9). The optical amplifier unit 140 amplifies the signal light output from the core unit 153 and outputs it to the optical transmission apparatus 103.
The optical amplifier unit 140 amplifies signal light received from the optical transmission apparatus 103 and outputs it to the core unit 154. The core unit 154 drops the signal light or adds signal light, and outputs the signal light to the optical amplifier unit 120 (although the optical transmission apparatus 102 does not drop the signal light or add signal light in the case of FIG. 9). The optical amplifier unit 120 amplifies the signal light output from the core unit 154 and outputs it to the optical transmission apparatus 101.
The OSC unit 151 monitors the optical network for an alarm or the like according to OSC light included in the signal light received from the optical transmission apparatus 101 and also monitors the optical transmission apparatus 102 itself for an alarm or the like. The OSC unit 151 outputs collected alarm information about the optical network and the optical transmission apparatus 102 itself to the optical transmission apparatus 101 through the optical amplifier unit 120.
The OSC unit 152 monitors the optical network for an alarm or the like according to OSC light included in the signal light received from the optical transmission apparatus 103 and also monitors the optical transmission apparatus 102 itself for an alarm or the like. The OSC unit 152 outputs collected alarm information about the optical network and the optical transmission apparatus 102 itself to the optical transmission apparatus 103 through the optical amplifier unit 140.
In the optical amplifier unit 120, a coupler 121 branches the signal light received from the optical transmission apparatus 101 and outputs it to a coupler 122 and a variable optical attenuator (VOA) 125.
The coupler 122 separates the signal light output from the coupler 121 into operating wavelength light and OSC light and outputs the operating wavelength light to a PD-SIG 123 and the OSC light to a PD-OSC 124. The PD-SIG 123 detects the level of the operating wavelength light, and the PD-OSC 124 detects the level of the OSC light.
The VOA 125 controls the attenuation of the signal light according to the level of light detected by a PD-VOA 126. A DCF 128 is connected between connectors 127a and 127b of the optical amplifier unit 120 to perform dispersion compensation of the signal light output from the PD-VOA 126.
A coupler 129 separates the signal light output from the DCF 128 into operating wavelength light and OSC light and outputs the operating wavelength light to a pre-amplifier module (PreAMPMd) 130 and the OSC light to the OSC unit 151. The OSC unit 151 is connected between connectors 132a and 132b of the optical amplifier unit 120.
In the OSC unit 151, a PD-OSC-U 133 detects the level of the OSC light input to the OSC unit 151. The optical transmission apparatus 102 uses whether the level of light is detected by the PD-OSC-U 133 to determine an unusual connection (such as a disconnected connector) of the DCF 128 and the OSC unit 151.
In the optical amplifier unit 120, a coupler 135 multiplexes operating wavelength light output from a post-amplifier module (PostAMPMd) 134 and the OSC light output from the OSC unit 151. The signal light obtained by this multiplexing is output to the optical transmission apparatus 101.
The PreAMPMd 130 amplifies the operating wavelength light output from the coupler 129 and outputs it to the core unit 153. The PreAMPMd 130 includes a PD-AMP 131 for detecting the level of the operating wavelength light at an input stage.
In the optical amplifier unit 140, a PostAMPMd 141 amplifies the operating wavelength light output from the core unit 153. A coupler 142 multiplexes OSC light output from the OSC unit 152 and the operating wavelength light output from the PostAMPMd 141. The signal light obtained by this multiplexing is output to the optical transmission apparatus 103. A coupler 143 separates the signal light received from the optical transmission apparatus 103 into operating wavelength light and OSC light, and outputs the OSC light to the OSC unit 152. A PreAMPMd 144 amplifies the operating wavelength light output from the coupler 143 and outputs it to the core unit 154.
Each of the PD-SIG 123, the PD-OSC 124, the PD-VOA 126, the PD-AMP 131, and the PD-OSC-U 133 is, for example, a tap photodiode (PD) or the combination of a PD and a coupler.
FIG. 11 illustrates the states of the optical transmission apparatus 102. FIG. 11 also illustrates whether each PD (each of the PD-SIG 123, the PD-OSC 124, the PD-AMP 131, and the PD-OSC-U 133) described with reference to FIG. 10 detects the level of light. In the FIG. 11, the symbol ◯ indicates that each PD detects the level of light and the symbol X indicates that the PD detects no level of light.
As indicated by No. 1, when each of the PD-SIG 123, the PD-AMP 131, and the PD-OSC-U 133 detects the level of light, the optical transmission apparatus 102 determines that it is in a usual operation state.
As indicated by No. 2, when each of the PD-SIG 123, the PD-AMP 131, and the PD-OSC-U 133 detects no level of light, the optical transmission apparatus 102 determines that it has an increased loss in a transmission line or has a low input level of the signal light (WDM-LOS).
As indicated by No. 3, when the PD-SIG 123 detects the level of light, but neither the PD-AMP 131 nor the PD-OSC-U 133 detects the level of light, the optical transmission apparatus 102 determines that it has an unusual connection with the DCF 128 or the OSC unit 151. In this case, however, the optical transmission apparatus 102 is unable to identify an unusual connection with the OSC unit 151 because it is impossible to determine whether an unusual connection with the DCF 128 or an unusual connection with the OSC unit 151 causes the PD-OSC-U 133 to detect no level of light.
As indicated by No. 4, when only the PD-OSC-U 133 detects no level of light, the optical transmission apparatus 102 determines that it has an unusual connection with the OSC unit 151 (OSC-LOS).
As indicated by No. 5, when only the PD-AMP 131 detects no level of light, the optical transmission apparatus 102 determines that it is in the WDM-LOS state or has a low input level at the PreAMPMd 130 (DCF-LOS).
As indicated by No. 6, when only the PD-OSC 124 and the PD-OSC-U 133 detect the levels of light, the optical transmission apparatus 102 determines that the DCF 128 has no unusual connection. Since the operating wavelength light is not used, that is, the zero-wave operation state occurs, in No. 6 and No. 7, the state of the optical transmission apparatus 102 is determined by whether the PD-OSC 124 detects the level of light. Since the operating wavelength light is not used, the PD-AMP 131 detects no level of light.
As indicated by No. 7, when the PD-OSC 124 detects the level of light, but neither the PD-AMP 131 nor the PD-SC-U 133 detects the level of light, the optical transmission apparatus 102 determines that the DCF 128 or the OSC unit 151 has an unusual connection. This is because it is impossible to determine whether an unusual connection with the DCF 128 or an unusual connection with the OSC unit 151 causes the PD-OSC-U 133 to detect no level of light.
The optical transmission apparatus 102 branches the OSC light at a subsequent stage of the DCF 128, and the PD-OSC-U 133 monitors the level of the OSC light to determine an unusual connection with the OSC unit 151. Therefore, as indicated in No. 3 and No. 7, when the PD-SIG 123 detects the level of light (No. 3) or the PD-OSC 124 detects the level of light (No. 7) but the PD-OSC-U 133 detects no level of light, the optical transmission apparatus 102 is unable to determine whether an unusual connection occurred with the DCF 128 or the OSC unit 151.
An optical amplification apparatus in which a DCF is connected between two amplifiers has been-proposed (for example, see Japanese Laid-open Patent Publication No. 2000-196169). An optical amplification apparatus that separates light into main signal light and a monitor control signal and detects the light level of the monitor control signal has also been proposed (for example, see Japanese Laid-open Patent Publication No. 2003-124891).
As described above, in conventional optical transmission apparatuses, unusual connections of components with a unit is unable to be identified.